1. Field of Invention
The invention relates to an Ethernet passive optical network ring used in a local optical network system. In particular, the invention relates to an Ethernet passive optical network ring that can prevent the whole system from breaking when the network ring fails without the need of additional active or passive optical devices in any form.
2. Related Art
The structure of a normal Ethernet passive optical network (EPON) ring is shown in FIG. 1. It uses an optical line termination (OLT) 11 to manage several optical network units (ONU) 12. The two ends of an optical ring 10 are connected to the OLT 11. Each of the ONU 12 uses an optical splitter 13 to connect to the optical ring 10. Therefore, controlled by the OLT 11, they can receive/transmit data. Such an optical network structure is usually suitable for systems with low-density users.
Due to intrinsic defects, the system breaks as the optical ring 10 fails. With reference to FIGS. 2A and 2B, the optical splitter 13 only provides one-way transmissions for the ONU 12. In other words, the OLT 11 can transmit data only in the clockwise direction along the optical ring 10 (see the arrows shown in FIGS. 1 and 2A), and receive data in the counterclockwise direction along the optical ring 10 (see the arrows shown in FIGS. 1 and 2B). Therefore, once the optical ring 10 breaks, as shown in FIG. 3, the downstream ONU also breaks. The drawing shows in order the first ONU 121, the second ONU 122, the third ONU 123, the fourth ONU 124, and the fifth ONU 125. When the optical ring 10 breaks between the third ONU 123 and the fourth ONU 124, both the fourth ONU 124 and fifth ONU 125 break.
To solve this problem, the U.S. Pat. No. 6,327,400 proposed a switching method to let the optical splitter couple to one end. When the network breaks, it provides a temporary solution. However, the switching method increases device costs and involves higher complexity in controls.
On the other hand, since the optical ring 10 only provides a single ONU 12 to transmit data at a time, collisions will happen when other ONU 12 are using the optical ring 10 to transmit data or perform authorization at the same time. Consequently, one needs to make collision detection beforehand to reduce collisions. With reference to FIG. 4, a 3×N optical splitter 142 connects two of the three ports on the left-hand side (LHS) using an isolator 141. When light sends data from the first ONU 121 at the port on the right-hand side (RHS) to the optical splitter 142, the connected ports on the LHS reflect the signal because of the isolator 141. The reflected signal passes through the optical splitter 142 and reaches the second ONU 122. The second ONU 122 has a wavelength division multiplexing (WDM) system 144, an optical receiving unit 142, a coupler 145, an optical transmitting unit 147, and a carrier sensor 146. The carrier sensor 146 receives the returned signal. The advantage of this method is to avoid collisions as a result of sending several signals at the same time the first ONU 121 transmits data. However, one has to include in addition a 3×N optical splitter 142 and an isolator 141. This does not only increase the cost but also makes the system structure more complicated.